Summary
This paper describes observations on water separation from wellstream in an
inline separator using the concept of distributed water withdrawal from an
inclined tube section. The physical observations were conducted in a laboratory
using a 7-in. observation section applying flow rates compatible to North Sea
single-well rates. The distributed water withdrawal is achieved by placing a
set of evenly spaced tapping points at the bottom of the tube section with
facilities to control tapping rates. The observations were made in connection
with a joint-industry project to validate and commercialize a new concept for
downhole and seabed separations. The driver for the study is to verify possible
water separation by distributed tapping from the bottom of the tube in high
flow rates with a high degree of agitation, turbulence, and phase dispersion.
Observations demonstrated that for a wide range of flow, comparable to the flow
conditions in high-rate oil wells, the high turbulence and phase dispersion do
not erode and disperse completely a continuous layer of water on the bottom of
the tube. This layer, flowing upward or downward depending on the flow
conditions, facilitates a reasonable water separation when it is tapped
properly. The scope of the test intended to capture the effect of flow and
system variables on the separation efficiency. The experiments and the analysis
validated the distributed separation concept. They demonstrated good separation
at a wide range of flow stream conditions. They explained the reasons for good
separation even with a high degree of turbulence and phase dispersion. The
mechanistic understanding of the separation allows a degree of predictability
and guidance for separator design.
Introduction
Inline water separation from wellstreams reduces the backpressure on flowing
wells, improves flow in gathering systems, and reduces water separation loads
in surface separators. It is widely recognized that separation close to the
source (the payzone) presents both hydraulic performance gain and phase
separation advantages. These rewards drive current development of several novel
inline separation concepts and separator designs, the reported concept
included. The discussed concept is based on gravitational separation in an
inclined tube with distributed water withdrawal. It appears to be effective,
with simple or no internals; it is robust in structure and has little
sensitivity to the accuracy of installation angle. It was initially conceived
for down-hole applications (Håheim 2003), but has promising prospects for
seabed separation in offshore fields.
Published information on inclined multiphase flow, developed primarily for
addressing pressure losses and transience in surface piping and wellbores,
failed to provide the information needed to predict distributed water
withdrawal from wellstream in inclined tubes. Therefore, a laboratory study has
been launched to study the relevant flow and separation aspects and to validate
the concept. The study included the design and construction of a dedicated test
loop intended to study the separation phenomenon with minimal needs for scaling
up the results to real well-stream conditions.
This paper presents the results of the laboratory test program, discusses
the process of validation of the separation concept, explains the observations
of the governing separation mechanisms, and assesses the importance of the
various involved parameters. It also provides preliminary information needed to
guide the design of a prototype for field trials.
The Separation Concept
The studied separation method uses an inclined separation tube where water
or water-reach phase is withdrawn in a distributed manner from the lower part
of the tube. Evenly distributed discrete tapping points located at the lower
end of the separation tube provide draining or withdrawal of water from the
higher pressure in the tube to a slightly lower pressure outside. Gravitational
forces, combined with the inclination effect of the upward flow, create
separated water or a water-reach layer at the lower side of the pipe. This
segregated layer is the core of the separation process. Depending on the flow
conditions, the layer can be relatively thick and flow upward or a thin film
and slide downward. Yet in a wide range of flow conditions, the water strata is
continuous and can be effectively tapped from the tube with careful and
properly adjusted distributed tapping. Fig. 1 illustrates schematically
the separation and tapping principle.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
30 September 2007
- Manuscript approved:
19 October 2007
- Version of record:
15 March 2008
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